1. Field of the Invention
The present invention relates to an optical film efficiently changing the optical path of incident light from a direction of a side surface into a viewing direction so that the optical film can be used to form a reflective liquid-crystal display device which uses a front light, which is thin in thickness and light in weight, and display of which is bright and easy to view.
The present application is based on Japanese Patent Application No. 2000-340415, which is incorporated herein by reference.
2. Description of the Related Art
An external light-illumination reflective liquid-crystal display device further reduced in thickness, size and weight is demanded in order to attain reduction in size and weight of a portable personal computer, a cellular phone, or the like. In the meantime, for a background-art liquid-crystal display device in which a front light using a side type light pipe is provided, however, it has been already difficult to reduce the thickness, size and weight of the liquid-crystal display device any more because a light pipe having a thickness of not smaller than 1 mm is required for light transmission. On the other hand, the same fact about the required thickness as described above is also true of a liquid-crystal display device in which a backlight using a semi-transmission type reflection plate is provided. In addition, in this case, there is a problem that brightness of the liquid-crystal display device using the backlight is inferior to that of an exclusive-use reflective liquid-crystal display device using a reflection layer high in reflectance. This problem is caused because the semi-transmission type reflection plate added to the display device makes bulk and weight larger and viewing in an illumination mode and viewing in an external light mode are dimmed due to dispersion of the light into transmitted light and reflected light through the semi-transmission type reflection plate.
It is an object of the present invention to develop an optical film changing the optical path of light incident from a direction of a side surface into that in a viewing direction efficiently so that the optical film can be used to form a reflective liquid-crystal display device which uses a front light and which is thin in thickness and light in weight, and display of which is bright and easy to view.
According to the present invention, there is provided an optical film constituted by: a transparent film including a repetitive structure of a plurality of light exit means arranged at intervals of a regular pitch or intervals of irregular pitches in one of opposite surfaces of the transparent film, the repetitive structure of the plurality of light exit means having optical path changing-slopes each inclined to the film surface at an inclination angle in a range of from 30 to 48 degrees in order to change an optical path of light from a direction of the film surface into a direction of thickness of the film; and an anti-reflection layer disposed on the film surface in which the plurality of light exit means are formed; wherein the optical film is light-transmissible in the direction of the thickness of the film. There is further provided a reflective liquid-crystal display device constituted by: a reflective liquid-crystal display panel including at least one light source on one of side surfaces of the liquid-crystal display panel; and the optical film provided on a viewing side of the reflective liquid-crystal display panel so that a surface of the optical film in which a plurality of light exit means are formed faces outward.
In the optical film according to the present invention, the optical film is disposed on the viewing side of a reflective liquid-crystal display panel having at least one light source on one of side surfaces of the liquid-crystal display panel. Hence, the optical path of light incident on the side surface or the optical path of transmitted light of the incident light is efficiently changed into a viewing direction of the liquid-crystal display panel through optical path changing slopes of the optical film, so that the light can be moved inversely after being reflected by a reflection layer. Thus, the light can be utilized for liquid-crystal display in an illumination mode. In addition, flat face portions are provided among the light path changing slopes of the optical film so that external light can be made incident on the optical film efficiently and then the incident light can be moved inversely after being reflected by the reflection layer. Hence, the light can be utilized for liquid-crystal display in an external light mode. As a result, the optical film has little surface reflection and glare of external light due to the surface reflection is prevented from being generated so that the optical film can be used to form an external light-illumination liquid crystal display device which is excellent in reduction in thickness and weight, which is bright and which is good in display quality.
The aforementioned effects chiefly depend on the optical film which makes use of reflection by the slopes to thereby control the optical path of the light. That is, light travelling in a direction of the surface of the film (light incident on the side surface or transmitted light thereof) is reflected by the optical path changing slopes so that the optical path of the light can be changed with good directivity in a direction of the thickness of the film (a viewing direction). Hence, good visibility can be attained in the illumination mode. In addition, the flat face portions can be disposed among the optical path changing slopes easily so that external light can be transmitted through the flat face portions. Hence, sufficient incidence of the external light can be ensured so that good visibility can be attained also in the external light mode. It is difficult to attain the aforementioned effects in a scattering reflection system through a roughened surface formed on a scattering sheet, or the like. Incidentally, Unexamined Japanese Patent Publication No. Hei. 5-158033 suggests a reflective liquid-crystal display device in which illumination light is made to enter a liquid-crystal display panel from one of its side surfaces and totally reflected by a viewing-side cell substrate so that the reflected light is scattered by a roughened-surface type reflection plate so as to be utilized for display.
In this case, however, the light available for display is light made to exit from the panel against the total reflection conditions by scattering. Scattered light generally exhibits a normal distribution with a peak in a regular reflection direction. Accordingly, the display light is inclined to the front (vertical) direction at an angle too large to be effectively utilized for display. As a result, the display is dimmed in the front direction. However, even if diffusing by the roughened-surface type reflection plate is intensified, the quantity of light confined in the panel to be made unable to exit from the panel is increased so that the display is still dimmed. It is therefore necessary in such a roughened-surface scattering reflection system to adjust the intensity of scattering to balance the intensity of scattering in an external light mode with that in an illumination mode. It is however difficult to make the intensity of scattering favorable to both external light and illumination modes because the intensity of scattering required in the former is incompatible with that required in the latter.
On the other hand, in the optical film which uses reflection by the slopes to control the optical path of the light according to the present invention, light exhibiting a peak in the regular reflection direction is chiefly used, and the optical path of the reflected light is controlled. Accordingly, directivity favorable to display, particularly directivity in the frontal direction can be given to the optical film easily so that bright display can be achieved in the illumination mode. Also in the external light mode, good balance between a state favorable to the illumination mode and a state favorable to the external light mode can be ensured easily by use of the flat face portions of the optical film excluding the optical path changing slopes of the light exit means. Further, the anti-reflection layer suppresses surface reflection of the optical film to improve display quality particularly in the external light mode. As a result, the optical film can be used to form an external light-illumination liquid-crystal display device which uses a front light and which is excellent in display quality.
Features and advantages of the invention will be evident from the following detailed description of the preferred embodiments described in conjunction with the attached drawings.